Luminescent materials, termed phosphors, have general utility in luminescent displays. A phenomenon common to all phosphors is their ability to emit photons that are visible to the human eye when the phosphors are excited to elevated energy levels. One excitation technique employed in many luminescent displays, including cathode ray tubes, vacuum fluorescent displays, and field emission displays, projects electrons through a vacuum onto a display screen containing the phosphors from an electron-emitting cathode positioned proximal to the display screen. The display screen has a glass substrate coated with a layer of the luminescent phosphors and a layer of a conductive material that serves as an anode. When the cathode is activated, the electrons travel from the cathode to the anode causing incident electrons to strike the intervening phosphor layer. The incident electrons are reflected, scattered or absorbed by the phosphors, thereby transferring energy to the phosphors, exciting the phosphors and advantageously causing them to emit visible light.
If the energy of the incident electrons is greater than a few tens of electronvolts, the incident electrons can also create a large number of secondary electrons within the phosphor layer. Some of these secondary electrons can escape the surface of the phosphor layer back into the vacuum, if the secondary electrons have sufficient energy to overcome the work function. The escaping secondary electrons can undesirably cause the floating surface of the phosphor layer to shift its potential when the number of secondary electrons escaping the surface of the phosphor layer exceeds the number of incident electrons striking the surface. A reduction in the potential of the phosphor layer results in a negative charge buildup on the screen that seriously diminishes the light output of the phosphors, producing unstable emissions thereof.
This problem is obviated in cathode ray tubes by aluminising the screen with an aluminum film to discharge spent electrons from the screen and reduce the negative charge buildup thereon. Aluminisation cannot be employed in field emission displays or vacuum fluorescent displays, however, because, unlike cathode ray tubes, the accelerating voltages of emitted electrons in field emission displays and vacuum fluorescent displays are generally relatively low and aluminum films typically absorb incident electrons having energies below 2 keV. An alternative solution to the problem of negative charge buildup on display screens is to integrate an inert non-luminescent conductor such as indium oxide within the phosphor layer of the screen. The inert conductor effectively discharges the spent electrons from the phosphor layer reducing the negative charge buildup on the screen. Unfortunately, the inert conductor dilutes the phosphor concentration of the phosphor layer, thereby diminishing its inherent luminescent efficiency and correspondingly diminishing the quality of the image produced by the display.
As such, a need exists for a display screen having utility in a luminescent display that effectively discharges spent electrons from the screen without unduly diminishing the inherent luminescent efficiency of the device. Accordingly, it is an object of the present invention to provide a phosphor composition for coating a screen of a luminescent display that has a relatively high luminescent efficiency over a broad range of operating voltages It is another object of the present invention to provide a phosphor composition for coating a screen of a luminescent display that effectively discharges spent electrons therefrom without unduly diminishing the relatively high inherent luminescent efficiency of the display or the quality of the display image. It is yet another object of the present invention to provide a display screen device for a luminescent display that effectively discharges spent electrons from the screen without unduly diminishing the inherent luminescent efficiency of the display. It is still another object of the present invention to provide a process for effectively discharging spent electrons from a display screen of a luminescent display without unduly diminishing the inherent luminescent efficiency of the display. These objects and others are accomplished in accordance with the invention described hereafter.